1. Field of the Invention
The present invention relates to ribbon microphones and ribbon microphone units.
2. Related Background of the Invention
A ribbon microphone includes a magnet for forming a magnetic field and a ribbon diaphragm, as the main component members. The magnet is disposed on both sides of the ribbon diaphragm to sandwich the same, whereby a magnetic field is formed between the magnets on the both sides. The ribbon diaphragm is given an appropriate tension and disposed within the magnetic field while the both longitudinal ends are held down. In response to a sound wave, the ribbon diaphragm vibrates within the magnetic field and thereby a current corresponding to the sound wave flows through the ribbon diaphragm and thus the sound wave is converted into an electric signal. Typically, an aluminum foil is used as the material of the ribbon diaphragm. Since aluminium has a small conductive resistance as compared with other metallic materials and also has a low specific gravity, aluminium is suitable as the ribbon diaphragm of the ribbon microphone.
An electric signal which the ribbon diaphragm generates cooperating with the field is weak. Accordingly, the ribbon microphone commonly known includes a transformer to boost the voltage of a weak electric signal. However, since the electric signal flows, as a weak electric signal as is, on an electric circuit from the ribbon diaphragm to the transformer, the conductive resistance needs to be reduced as much as possible. If the conductive resistance is large, the sensibility of the electric signal will decrease and the expected acoustic characteristics cannot be obtained.
FIG. 5 shows an example of a typical ribbon microphone. FIG. 6 shows an example of a ribbon microphone unit built in this ribbon microphone. In FIG. 5, in a ribbon microphone 1, a housing of the microphone comprises a cylindrical base 6 and a microphone case 2 connected to the top end of the base 6. Inside this housing, a ribbon microphone unit 3 is assembled to a suitable support member that is secured to the base 6. The microphone unit 3 is covered with the microphone case 2. The bottom end of the base 6 is a connector portion 16, to which a microphone cable for delivering the output signal of the microphone to an external circuit is connected.
As also shown in FIG. 6, the ribbon microphone unit 3 includes a frame 7 formed in a rectangular frame shape that is long in the longitudinal direction. On both sides of the inner surface of the frame 7, a pair of permanent magnets 4, 4 are fixed along the longitudinal direction, with a predetermined spacing between the both permanent magnets 4, 4. The permanent magnets 4, 4 are magnetized in the width direction (horizontal direction in FIG. 6), and the directions of the magnetization of the pair of permanent magnets 4, 4 are the same. Accordingly, the parallel field is formed between the permanent magnets 4, 4.
Within the parallel field, a ribbon diaphragm (hereinafter, may be referred to simply as a “ribbon”) 5 combining a diaphragm with a conductor is disposed. As shown in FIG. 10, the ribbon 5 forms a long and slender band shape, and both longitudinal ends thereof are secured to electrode lead portions 18, 18 provided on both longitudinal ends of the frame 7. The electrode lead portions 18, 18 are insulated from the frame 7, and are conducted to the ribbon 5 by sandwiching both ends of the ribbon 5 using tightening members comprising pressure plates 8, 8 and support plates 81, 81. Moreover, the electrode lead portions 18, 18 hold the ribbon 5 in a state where the ribbon 5 is given an appropriate tension. In the ribbon 5, portions 51, 51 other than the portions corresponding to the electrode lead portions 18, 18 are formed in the shape of a triangular wave by being alternately folded at fixed intervals. The direction of lines formed by the folding, i.e., the direction of the lines which the top and the bottom of the triangular wave draw are the width direction of the ribbon 5, and these lines are formed at fixed intervals. Hereinafter, both corrugated ends of the ribbon 5 are referred to as a corrugated end 51, 51, respectively. Terminal strips 9, 9 are overlapped onto the electrode lead portions 18, 18, respectively, and onto each terminal strip 9 the pressure plate 8 is overlapped sandwiching the end of the ribbon 5. These terminal strips 9, 9 are electrically conducted to each end of the ribbon 5 via the pressure plates 8, 8 so that a signal from the ribbon microphone unit 3 may be output from each of the terminal strips 9, 9. The intermediate portion in the longitudinal direction of the ribbon 5 is a corrugated intermediate portion 52, in which the top and the bottom of a triangular wave are formed along the line (i.e., line in the longitudinal direction of the ribbon 5) in the direction perpendicular to the direction of lines which the top and the bottom of the triangular wave of the corrugated ends 51, 51 draw. The both longitudinal ends 54, 54 of the ribbon 5 are flat, not having corrugated irregularities formed therein.
The ribbon 5 vibrates in response to a sound wave and in accordance with the sound wave. This vibration direction is the direction intersecting the magnetic flux between the permanent magnets 4, 4, and the ribbon 5 made of a conductor intersects the magnetic flux to thereby generate electric power, whereby an electric signal is generated between the both longitudinal ends of the ribbon 5, accordingly between the electrode lead portions 18, 18. Since this electric signal turns into a signal having a frequency and amplitude corresponding to the frequency and amplitude of the ribbon 5, a sound wave which hits the ribbon 5 will be converted into an electric signal corresponding to the sound wave. Since the ribbon microphone is an inertia control system, the resonance frequency of the ribbon 5 needs to be no greater than the low frequency of sound waves to be collected, in other words, needs to be a frequency lower than the lowest frequency of a frequency band in which sounds can be collected. For this reason, the tension of the ribbon 5 is set extremely low. As described above, in the ribbon 5, the corrugated ends 51, 51 are formed by the both ends being folded, whereby a low tension is realized.
In the example of the ribbon microphone described above, the intermediate portion in the longitudinal direction of the ribbon 5 is the corrugated intermediate portion 52, in which the top and the bottom of a triangular wave are formed along the longitudinal line of the ribbon 5. The ribbon of most of the conventional ribbon microphones is formed so that the direction of the lines, which the top and the bottom of the triangular wave draw, is the width direction of the ribbon, and the triangular wave travels across the longitudinal direction. FIG. 7, FIG. 8, and FIG. 9 show enlarged views of the electrode lead portion 18 having the ribbon diaphragm 5 and the pressure plate 8 in the ribbon microphone unit shown in FIG. 6. In FIGS. 7A and 7B, the both longitudinal ends of the ribbon diaphragm 5 are sandwiched by the support plates 81, 81 and the pressure plates 8,8, and are secured by fastening the both ends of the pressure plate 8 to the support plate 81 with screws 10, 10 respectively. As shown in FIG. 8, when the screw 10, 10 are tightly fastened, the pressure plate 8 is bend-deformed so as to surround the ribbon diaphragm 5 in the width direction, in other words, the pressure plate 8 is bend-deformed in such direction that the both ends rather than the center portion in the width direction of the diaphragm 5 come closer to the diaphragm 5. Because the pressure plate 8 is bend-deformed in this manner, the pressure in the center portion in the width direction of the ribbon diaphragm 5 is weak and only the both ends contact the pressure plate 8. Accordingly, the conductive resistance of the ribbon diaphragm 5 will increase. Then, as shown in FIG. 9, the pressure plate 8 is bent in a direction opposite to the above-described bending direction in advance, so that the ribbon diaphragm 5 may uniformly contact the pressure plate 8 when the both ends of the opposing pressure plate 8 are fastened. However, in order to deform the pressure plate 8 so that the both ends of the ribbon diaphragm 5 become flat and uniformly contact the pressure plate 8, expert skills and experiences of the operator are required. Moreover, even if the pressure plate 8 is deformed as described above, problems as described below cannot be resolved.
As shown in FIG. 10, the both longitudinal ends 54, 54 of the ribbon diaphragm 5 are flat planes. In the maintenance of the ribbon microphone, when replacing the ribbon diaphragm 5, as shown in FIG. 6, the both ends of the ribbon diaphragm 5 should be disposed in the electrode lead portions 18, 18 that are formed in the frame 7 of the ribbon microphone unit 3. However, since the both ends 54, 54 of the ribbon diaphragm 5 are flat planes, they are elastically weak and wobble and thus the position in attaching the both ends 54, 54 to the electrode lead portion 18 becomes unstable, and the attachment work of the diaphragm becomes difficult. Moreover, if the both ends of the ribbon diaphragm 5 are disposed in the electrode lead portions 18, 18 and then fastened with the screws 10, 10 via the pressure plate 8, a stress caused by fastening the screws 10, 10 transmits to the pressure plate 8 and twists the ribbon diaphragm 5. This stress on the diaphragm does not allow the diaphragm to vibrate faithfully to sound waves. If a microscopic rotational stress due to the screw occurs, the diaphragm assembly work needs to be done again. The rate that the assembly should be carried out again is called a failure rate, and the shape of the conventional ribbon diaphragm has a drawback of increasing the failure rate at the time of assembling.
Next, the invention described in a patent document related to the present invention is described. The invention described in Patent Document 1 relates to a method of manufacturing a ribbon microphone, and in particular, relates to a method of manufacturing the ribbon microphone capable of adjusting the tension of the ribbon diaphragm while observing the same. In the ribbon diaphragm described in Patent Document 1, both longitudinal ends are flat planes, and corrugated irregularities are formed continuously in an intermediate portion in the longitudinal direction. The both ends of the ribbon diaphragm are flat planes, and the both flat ends are crimped between presser plates and support plates by fastening screws, respectively.    [Patent Document 1] Japanese Unexamined Patent Application Publication No. 2006-319595